Systems for measuring properties of products contained in tanks or vessels—so-called tank gauging systems—are ubiquitous in application areas involving handling, shipping and storing of products as well as, for example, in the chemical process industry.
Since products to be monitored and/or measured are often hazardous, special safety requirements exist for equipment, such as tank gauging systems or at least parts thereof that are positioned within a so-called hazardous area. Therefore, such equipment generally needs to be certified as either explosion-proof or intrinsically safe. For intrinsically safe equipment, there are limitations to ensure that the equipment is unable to cause ignition of a gas, which may be present in the hazardous area.
A representative area of application of tank gauging systems is in a storage facility for petroleum products and the like, often referred to as a “tank farm”. In such a tank farm, each tank is typically equipped with a number of sensing units, each configured to sense a certain property, such as level, temperature, pressure, etc of the product contained in that tank.
Traditional intrinsically safe systems for hazardous environments are mainly analog so-called 4-20 mA systems, in which sensing units are connected in a point-to-point fashion to a central host via intrinsically safe barriers in order to provide intrinsic safety within the hazardous area.
It is easily understood that traditional 4-20 mA systems require a great deal of wiring. Especially for an application such as a tank farm in which the tanks can be separated by considerable distances, the wiring, together with the large number of intrinsically safe barriers needed, stands for a substantial portion of the cost of installing the tank gauging system.
One method of reducing the amount of wiring in an intrinsically safe system is to use a digital intrinsically safe communication bus. Using such a bus, various sensors may be connected along the bus, and it is sufficient to route one cable from a number of sensors to a control room. An example of such a digital communication bus is the HART-bus where up to 15 sensors can be connected on one bus segment. Another method of reducing the amount of wiring in an intrinsically safe system is to use wireless technologies for communicating with the sensing units. For example, completely wireless installations are used in which the field device uses a battery, solar cell, or other technique to obtain power without any sort of wired connection.
Another example is provided through U.S. Pat. No. 7,262,693, disclosing a combination of wired and wireless communication with a sensing unit. In this example, an intrinsically safe control loop carries data and provides power to a wireless field device connected in series with the sensing unit, and RF circuitry in the wireless field device is powered using power received from the intrinsically safe two-wire process control loop. The wireless field device is further adapted to limits its influence on the two-wire process control loop.
However, the wireless field device in some cases provides limited possibilities to wirelessly transmit and receive information, as the intrinsically safe two-wire process control loop is strictly restricted in the sense of how much power that can be provided to the wireless field device without severely influencing information communicated over the two-wire process control loop.
There is thus a need for an improved gauging system having wireless capabilities.